2 part foam??

ColumbiaNX01 · Dec 10, 2016

2 Part expanding foam. I have used lots of it before. I have some leftover 2 pound. But what is the true advantages of using 4 lb, 6 lb, or 8lb. All these weights you can get through rocket vendors. Obviously, the heavier foam is tougher and harder. When foaming the inside a fin can between airframe and motor tube are there any advantages or disadvantages of using either kind?

thanks

mpitfield · Dec 10, 2016

I think you answered your own question, weight vs. strength. I have only used the PML 2 part foam, which does allow you to adjust the strength/weight by adding drops of water, however I have only used it full strength. Well I did try to change the density once however that resulted in a mess because I was not capable of following the instructions and the foam ended up far too light...when they say a drop they mean a drop or two.

Having said that at this stage I would be more inclined to use internal fillets if over expanding foam, unless there was a compelling reason to use the foam instead. I have only come to this conclusion recently after trying to service/rebuild a couple of rockets that had foamed cans and the foam added to the work, although it was not the end of the world in my case.

ColumbiaNX01 · Dec 10, 2016

mpitfield said:
I think you answered your own question, weight vs. strength. I have only used the PML 2 part foam, which does allow you to adjust the strength/weight by adding drops of water, however I have only used it full strength. Well I did try to change the density once however that resulted in a mess because I was not capable of following the instructions and the foam ended up far too light...when they say a drop they mean a drop or two.

Having said that at this stage I would be more inclined to use internal fillets if over expanding foam, unless there was a compelling reason to use the foam instead. I have only come to this conclusion recently after trying to service/rebuild a couple of rockets that had foamed cans and the foam added to the work, although it was not the end of the world in my case.

Is there a formula to figure how much the foam will weigh? If I use 8 lb foam vs. 2 lb foam in a certain area it will tell me what it will weigh when cured?

mpitfield · Dec 10, 2016

ColumbiaNX01 said:
Is there a formula to figure how much the foam will weigh? If I use 8 lb foam vs. 2 lb foam in a certain area it will tell me what it will weigh when cured?

I just did a quick Google and some of the 2lb, 4lb, etc. two part foams include a specification sheet with a cure density PCF. If you can find the cure density PCF for the product then some quick math should tell you the delta.

sharkbait · Dec 10, 2016

The foams are their expanded cured weight, a 2lb foam weighs 2lbs per cubic foot, 6 lb foam weighs 6lbs per cubic foot.

Simply calculate the area you are foaming in cubic inches, convert that to cubic feet and you will have your weigh in pounds, which can be converted to ounces or grams or whatever you want.

IMHO check out U.S. Composites, their foam is top shelf and available in 2, 4, 6, 8, & 16 pound formulas

MikeyDSlagle · Dec 10, 2016

Use the formula to find volume of a cylinder (Pi x radius square x height). Your height will of course be the distance between your centering rings, err well it usually is. Use ID of body tube to determine radius. Then figure the volume of your MMT using the same height and the OD to figure radius. Then subtract MMT volume from BT volume. Convert to whatever units you prefer.

I think that's right lol

And +1 to US Composites, I use a lot of their stuff. Great customer service too.

Mikey D